1. Field of the Invention
The present invention generally relates to wireless Code Division. Multiple Access (CDMA) systems and, more particularly, to automatic frequency control (AFC) in CDMA systems with constant pilot signals.
2. Background of the Invention
In passband CDMA systems, the receivers usually need to generate a local carrier to demodulate the received signal. An automatic frequency control (AFC) operation of the CDMA systems generates a local carrier using a frequency close to the transmitter carrier frequency, and also tracks the deviation of the local frequency from the transmitter frequency distorted by channel propagation. For CDMA systems, such as, for example, Interim Standard-96 (IS-95) and Universal Mobile Telecommunications System (UMTS), a constant pilot signal is used to perform the AFC operation.
In CDMA systems, the pilot signal used for the AFC operation is demodulated to the in-phase and quadrature baseband signal using the generated local carrier. However, the frequency of the local carrier is usually different from the frequency of the transmitter carrier. In UMTS systems, due to component inaccuracies and channel variations, the frequency difference could be as high as 10 KHz, which can have the effect of severely degrading the receiver performance. An AFC loop is therefore exploited to reduce the frequency difference to a level that introduces only a negligible performance loss.
In AFC operation, a feedback loop is formed, commonly referred to as an AFC loop. In this loop, an error signal representing the difference between the local carrier and the transmitter carrier frequency is computed and filtered. The filtered output is then used to adjust the local carrier frequency. As such, the quality of the error signal directly affects the performance of the AFC loop.
In a conventional AFC loop in CDMA systems, pilot signals are first despread to obtain despread symbols. The phase difference between the current despread symbol and the previous despread symbol is computed, representing the residual frequency offset between the local carrier and the transmitter carrier. The phase difference signal is filtered using a first-order or second-order loop filter in order to generate the error signal. However, due to the severe fading effect in wireless channels, this type of error signal usually has a large variation, even after the filtering operation, and thus can make the AFC loop unstable. Many proposed AFC loops are not robust in the presence of mobile channel degradations.
Accordingly, it would be desirable and highly advantageous to have an AFC loop for a CDMA system that overcomes the above-described problems of the prior art.